Add Prompt canceled delimiter to GitLab DuoChat

Description

In order to more visibly indicate to a user that the prompt has been cancelled (especially if the response has not started streaming yet), a Prompt canceled delimiter should be added.

There are some prior threads here discussing the implementation. Similar to the /reset slash command, the implementation for this work would be very similar.

Checklist

Make sure the following are completed before closing the issue:

1. Assign the correct component label to this issue.
2. Create an MR with the additions or updates needed.
3. Be sure to get your MR reviewed by a FE/UX Foundations designer.
4. When applicable, create an MR in Pajamas to update the demos. If you do not have capacity to complete the demos, create an issue in Pajamas and bring the issue to your team planning session for prioritization and scheduling. Mark the issue as related to this one.
5. When applicable, create an MR in Pajamas to update the component status.
6. When applicable, create an issue in Pajamas using the "Component Documentation" issue template to update guidelines. Ping a designer on your team for awareness and bring the issue to your team planning meeting for prioritization and scheduling.
7. When applicable, create an issue in Pajamas using the "Figma update" issue template to update the Figma UI Kit. Ping a designer on your team for awareness and bring the issue to your team planning meeting for prioritization and scheduling.
8. When introducing a major or breaking change, communicate the changes within the Engineering Week in Review.
9. 🎉 Congrats, you made it! You can now close this issue.